1. Nigel Roberts Met Office @ Reading
Predictability in a convection-permitting model
COPE-ing with uncertainty
Nigel Roberts Met Reading
With slides taken from work by Kirsty Hanley and Seonaid Dey
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2. MOGREPS-UK Convection-permitting 2.2 km ensemble now running routinely
Downscaling within MOGREPS-R ensemble members (18 km)
12 members
36 hour forecast every 6 hours
No high-resolution initial perturbations or ‘forecast error’ perturbations at present
Change to direct nesting in ~33 km global MOGREPS-G
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3. Example
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4. Want to understand the nature of the predictability of convective storms in a convection-permitting ensemble
An ensemble should represent the ‘true’ forecast uncertainty– for useful warnings of high impact weather
- Larger-scale environment
- Local effects and ‘model error’ (model physics, stochastic backscatter, surface effects …) – importance of storm-scale feedbacks
Require observations to assess physical processes in the model.
Local feedbacks may be important for high-impact localised rainfall totals.
Ensemble test-bed for evaluating model behaviour or biases and testing changes.
Not effective without knowledge of the ‘truth’

5. Small uncertainty at large scales = large uncertainty at small scales
Low
Noise at smallest scales ?
5% error at 1000 km = 100% error at 50 km
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6. LBC driven uncertainties
COPS IOP 8b at 12 UTC on July 27th 2007
Kirsty Hanley et al (2011) QJ
PVU. Solid: six strongest convection members.
Dashed: six weakest convection members (dashed)
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7. Physics vs. boundaries Seonaid Dey and Giovanni Leoncini FSS
FSS for precipitation hourly accumulations
FSS
Values 0-1
1 = ‘perfect match’
0 = ‘totally different’
Contours every 0.1, colours black at 0.0 to red at 1.0
Graupel / convection scheme / timestep had little effect at reliable scales
Time after start
gridlengths
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8. Outflow convergence

9. Ensemble comparison of microphysics schemes
Clark et al, 2012 BAMS
Composite frequencies of observed rainfall greater than 0.50-in. relative to grid-points forecasting rainfall greater than 0.50-in. at forecast hour 30 from SSEF members using (a) Thompson, (b) WSM6, (c) WDM6, and (d) Morrison microphysics parameterizations. The boldface dot in each panel marks the centre of the composite domain and the location of the observation.

10. Summary Current status
Routinely running 12 member 2.2 km ensemble 4 times a day
Major questions
What governs predictability in convection-permitting models ?
How does ensemble evolution relate to the real atmosphere ?
What model perturbations impact the representation of uncertainty for high-impact rainfall (physics, stochastic, surface …) ?
Datasets and model improvements
Observations of storm environment and local physical processes
Ensemble framework to determine model deficiencies and test changes
Improve ensemble perturbations to give more useful warnings

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